A colloidal light valve is a device whose light transmission characteristic is a function of the condition or state of an independent control variable. The light valve typically comprises a matrix of a host material containing a suspension of dispersed dipolar particles. Each dipolar particle will tend to permit the transmission of light through the valve when it has a first orientation with respect to a path of light travel, and tend to block the transmission of light through the valve when it has a second orientation with respect to the path of light travel. The orientation of each dipolar particle is a function of the state or condition of the control variable. The control variable is commonly either an electrical or magnetic field applied through the light valve.
The following references disclose various forms of light valves utilizing a suspension of dipolar particles: Land, U.S. Pat. No. 1,955,923; Marks, U.S. Pat. No. 3,527,525; Marks, U.S. Pat. No. 3,709,828; Marks, U.S. Pat. No. 3,773,685; Marks, U.S. Pat. No. 3,813,265; Yazaki et al, U.S. Pat. No. 3,823,998; Marks, U.S. Pat. No. 3,841,732; Marks, U.S. Pat. No. 3,848,964; and Marks, U.S. Pat. No. 3,900,417.
In many of the above cited references, the light valve takes the form of a dispersion of elongated electric dipoles in a liquid medium having a relatively high electrical resistance. The liquid is confined in a thin layer between a pair of planar, parallel, transparent, conductive electrodes. When a voltage is impressed between the electrodes, the elongated dipoles align and permit the transmission of light through the light valve. When the voltage is removed, the dipoles, through Brownian motion, orient randomly and the light valve becomes virtually opaque. One limitation of light valves of this type follows from the tendency of the dipoles to settle from their dispersed condition, especially at temperature extremes. Another limitation of a light valve of this type is that it is not amenable for use in display using matrix address techniques.
An improved form of light valve contemplates the use of spherical dipolar particles suspended in a matrix. The matrix material is plasticized to allow the cavities in which the spheres are located to expand. The differential increase in volume due to plasticizing becomes filled with the plasticizing agent. In this manner, the spheres are free to rotate within their volumes, yet are caged therein to prevent settling or agglomeration. This general type of light valve is disclosed in Sheridon, "The Gyricon-A Twist Ball Display", paper 9.6 at the 1977 Conference of the Society of Information Display.
The light valve disclosed by Sheridon uses a white/black sphere as the dipolar element or particle. In particular, one hemispherical surface is white, and the other hemispherical surface is black. The alignment of the spheres is controlled by an electric field that acts upon the different electrophoretic potentials associated with each of the different colored halves of the sphere. This type of light valve changes reflectivity with alignment of the spheres, but not degree of transparency as is generally the case with the more commonly known types of elongated dipole particle dispersion.